1. Field of the Invention
The present invention relates to circuit interrupters generally and, more specifically, to those kinds of circuit interrupters having a trip mechanism including an automatic trip assembly with a bimetal for generating a thermally-induced tripping operation.
2. Description of the Prior Art
Molded case circuit breakers and interrupters are well known in the art as exemplified by U.S. Pat. No. 4,503,408 issued Mar. 5, 1985, to Mrenna et al., and U.S. Pat. No. 5,910,760 issued Jun. 8, 1999 to Malingowski, et al., each of which is assigned to the assignee of the present application and incorporated herein by reference.
A continuing industry objective with respect to many types of circuit interrupters is to be able to reduce the size and/or footprint of the interrupter housing while at the same time providing the same or improved performance capabilities. A major advantage of creating such a "smaller package" is that it provides increased flexibility in installation. However, a consequence of this objective is that the internal space constraints of such interrupters have become much more limiting, posing certain design obstacles that need to be overcome.
Circuit interrupters advantageously provide for automatic circuit interruption (opening of the contacts) when an overcurrent condition is determined to exist. One way of determining whether or not an overcurrent condition exists is to provide a trip mechanism with an automatic trip assembly having a bimetal through which current flows. The bimetal reacts to overcurrent conditions by heating up and bending so as to set in motion a tripping operation. Another way of determining whether or not an overcurrent condition exists is to enable the automatic trip assembly to react to a magnetic field generated by the condition. The reaction to the magnetic field is often in the form of a movement of an armature that, in turn, sets in motion a tripping operation. The movement of the armature normally is either away from or towards a magnetic structure from which the magnetic field emanates.
In some circuit interrupters, both a thermally-induced and a magnetically-induced tripping operation is provided. In such interrupters, the bimetal is frequently positioned such that a portion thereof extends between portions of the magnetic structure. This "extending portion" of the bimetal is normally thinner than the rest of the bimetal in order to provide for a proper fit between the magnetic structure portions. In the prior art, the bimetal configuration of such interrupters does not provide for clearance of other internal circuit components, resulting in a bimetal that occupies more internal space than desired. The prior art bimetal configuration also suffers from current flow difficulties near the top of the thinner "extending portion" due to the narrowing of the flow path of current. These flow difficulties can increase the current density to the point where the bimetal material could undesirably melt and erode.
It would be advantageous if a bimetal configuration existed that could provide clearance for other internal components and therefore aforementioned space constraints. It would also be advantageous if a bimetal configuration existed that did not suffer from current flow difficulties that could lead to melting and erosion.